An I/O-bus network expansion system is disclosed in Patent Document 1. As shown in FIG. 9, this I/O-bus network expansion system includes computers 8a and 8b, a system manager 30 and an IO-device enclosure 9 that are connected via a network 2.
The computer 8a includes a CPU 81a that runs a program, a memory 83a that holds the program and data, a computer's I/O-bus-and-network bridge 84a that encapsulates an I/O packet into a network packet and transmitting to or receiving from the network 2, and a bridge 82a that connects the CPU 81a, the memory 83a and the computer's I/O-bus-and-network bridge 84a with each other. The bridge 82a and the computer's I/O-bus-and-network bridge 84a are connected by an I/O bus.
The computer 8b has the same configuration as the computer 8a described above, and has the same components 81b to 84b as the components 81a to 84a described above. Therefore, a detailed description of the components 81b to 84b will be omitted.
The network 2 transmits a network packet in which an I/O packet is encapsulated, to a target node. For example, the network 2 transmits a network packet from the computer 8a to an I/O device 92 or from the I/O device 92 to the computer 8a. 
The IO-device enclosure 9 includes an enclosure's I/O-bus-and-network bridge 91 that encapsulates an I/O packet into a network packet and transmitting to or receiving from the network 2, and the I/O device 92. The enclosure's I/O-bus-and-network bridge 91 and the I/O device 92 are connected by an I/O bus. The I/O device 92 is, for example, a network interface that is compliant with PCI Express (PCIe), or a combination of a storage controller and a hard disk.
The system manager 30 controls allocation of the I/O device 92 between the computer 8a and the computer 8b. To be specific, the system manager 30 issues a control packet to the enclosure's I/O-bus-and-network bridge 91 and thereby changes a computer to which the I/O device 92 is allocated. A case of changing a destination of allocation of the I/O device 92 from the computer 8a to the computer 8b will be considered as an example. In this case, the system manager 30 issues a control packet to the enclosure's I/O-bus-and-network bridge 91, and thereby, the enclosure's I/O-bus-and-network bridge 91 terminates connection with the computer's I/O-bus-and-network bridge 84a of the computer 8a and newly establishes connection with the computer's I/O-bus-and-network bridge 84b of the computer 8b. 
The I/O-bus network expansion system having the abovementioned configuration operates in the following manner. Below, a case in which the I/O device 92 is allocated to the computer 8a will be described.
First, a case of transmitting an I/O packet from the computer 8a to the I/O device 92 will be described. Such communication occurs when the program running on the CPU 81a accesses the I/O device 92, or when the I/O device 92 retrieves data from the memory 83a by DMA (Direct Memory Access). At this moment, the bridge 82a issues an I/O packet for the I/O device 92.
Then, the computer's I/O-bus-and-network bridge 84a encapsulates the received I/O packet into a network packet, and transmits this network packet to the network 2 with the network address of the enclosure's I/O-bus-and-network bridge 91 as a destination. The network 2 transmits the network packet that the I/O packet is encapsulated, to the enclosure's I/O-bus-and-network bridge 91. Then, the enclosure's I/O-bus-and-network bridge 91 receives and decapsulates the network packet that the I/O packet is encapsulated, and transmits the I/O packet to the I/O device 92.
Also in an operation of transmitting an I/O packet from the I/O device 92 to the computer 8a, the I/O packet is encapsulated into a network packet in the same manner as in the operation of transmitting an I/O packet from the computer 8a to the I/O device 92. Then, the I/O packet received by the bridge 82a is transmitted to the CPU 81a, or transmitted to the memory 83a by DMA.
While the I/O device 92 is allocated to the computer 8a as described above, an access from the computer 8b to the I/O device 92 does not occur.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-219873
However, the network expansion system having the abovementioned configuration has the following problems. A first problem is that the I/O device 92 cannot be controlled from the computer 8a or 8b. This is because a network packet transmitted from the computer 8a, 8b is a network packet in which an I/O packet is encapsulated and it is therefore impossible to 15 perform I/O-device control that is not defined in the I/O packet. Likewise, a second problem is that the computer cannot be controlled from the I/O device. This is because a network packet transmitted from the I/O device 92 to the computer 8a, 8b is a network packet in which an I/O packet is encapsulated.
On the other hand, when controlling the I/O device from the computer or when controlling the computer from the IO-device enclosure, there is a need to connect the computer and the system manager or connect the IO-device enclosure and the system manager by a management network of another system. Consequently, there arises a problem such as a complicated configuration of the system and increase of the cost.